Dipole moments charge transfer contributions

Fig. 13 Comparison of the HI and HIOS a spin component of the dipole moment of PF ( 2 ) calculated with the CASSCF(12,8) method using the av5z basis and partitioned into the charge transfer contribution q(F)R and the induced atomic dipoles /t(F) and /t(P)...

Atomic-level response of sodium clusters to external electric field has also been studied recently [55] using a decomposition of the total cluster dipole moment and polarizability into contributions from atomic volumes. The atomic dipole moments and polarizabilities thus obtained have also been partitioned into the atomic dipole and charge-transfer components. The relative contribution of these two components as a function of the size and shape of the clusters have been studied. Also the contributions are shown to depend on the location of the atomic site in the cluster. Thus, the surface atoms have been shown to have larger contribution to the polarizability than the interior ones. The anisotropy of the total polarizabilities is also shown to correlate with the shape anisotropy of the clusters. These ab initio results on the atomic charge and dipole components that contribute to the overall cluster polarizability thus would enable one to validate the coarse-grained DFT-based results in a more detailed fashion since the coarse-grained approaches provide the atomic charges and atomic dipoles besides the overall polarizability. 

Another example of the importance of atomic dipoles appeared in Chapter 2, where we attributed the small dipole moment of NF3 to the moment produced by the lone pair on nitrogen, which makes an important contribution to the atomic dipole on nitrogen and opposes the charge transfer moment due to the electronegativity difference between nitrogen and fluorine. 

Based on the fundamental dipole moment concepts of mesomeric moment and interaction moment, models to explain the enhanced optical nonlinearities of polarized conjugated molecules have been devised. The equivalent internal field (EIF) model of Oudar and Chemla relates the j8 of a molecule to an equivalent electric field ER due to substituent R which biases the hyperpolarizabilities (28). In the case of donor-acceptor systems anomalously large nonlinearities result as a consequence of contributions from intramolecular charge-transfer interaction (related to /xjnt) and expressions to quantify this contribution have been obtained (29). Related treatments dealing with this problem have appeared one due to Levine and Bethea bearing directly on the EIF model (30), another due to Levine using spectroscopically derived substituent perturbations rather than dipole moment based data (31.) and yet another more empirical treatment by Dulcic and Sauteret involving reinforcement of substituent effects (32). 

An ideal polarity probe based on photoinduced charge transfer and solvent relaxation should (i) undergo a large change in dipole moment upon excitation but without change in direction, (ii) bear no permanent charge in order to avoid contributions from ionic interactions, (iii) be soluble in solvents of various polarity, from the apolar solvents to the most polar ones. 

It should be mentioned that one would not expect the Eb, Cb, Ea and Ca parameters to be related to the ground state properties of the donor and acceptor. For example, although BF3 is a planar molecule with no dipole moment, there will be considerable contributions to the bonding in a BF3 adduct from dipole-dipole interactions. In the adduct, the geometry of the BF3 part is such that this fragment is very polar. Consequently, its Ea parameter will be appreciable. One must also consider that the lone pair dipole moment of the donor is modified upon adduct formation by the amount of charge transfer that takes place and somehow the parameters and Eq. (13) take this into account. 

V. The largest work function change was observed during the adsorption of pyridine (—2.7 V) and reflects the large contribution of the nitrogen lone electron pair and/or the permanent dipole moment to the charge transfer. 

The first term on the right, y, is the electronic contribution of 7° to the polarization at 2o) and the second term the contribution from pz. Note that pz cannot be determined from this experiment without a knowledge of the dipole moment. In compounds exhibiting significant charge transfer resonance pzPz y and the contribution of y is often ignored. 

The dielectric constant is a macroscopic property of the material and arises from collective effects where each part of the ensemble contributes. In terms of a set of molecules it is necessary to consider the microscopic properties such as the polarizability and the dipole moment. A single molecule can be modeled as a distribution of charges in space or as the spatial distribution of a polarization field. This polarization field can be expanded in its moments, which results in the multipole expansion with dipolar, quadrupolar, octopolar and so on terms. In most cases the expansion can be truncated to the first term, which is known as the dipole approximation. Since the dipole moment is an observable, it can be described mathematically as an operator. The dipole moment operator can describe transitions between states (as the transition dipole moment operator and, as such, is important in spectroscopy) or within a state where it represents the associated dipole moment. This operator describes the interaction between a molecule and its environment and, as a result, our understanding of energy transfer. 

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